In nuclear power plants, it is customary to provide a pool of water for the purpose of cooling spent fuel assemblies, which are immersed in the pool of water. It is necessary to circulate and cool the water of the pool, and in most installations, a water-water heat exchanger is used to cool both the water from the spent fuel pool and the water that is used to cool components of the reactor. The component cooling water is circulated through the water-water heat exchanger through which the water from the pool is also circulated.
Such water-water heat exchangers are relatively large and expensive, and the ultimate cooling therefor is obtained from a service water system, such as a river, lake or other source. If there is a loss of supply of water from the service water system, or if the water-water heat exchanger becomes inoperative, the spent fuel cooling system is inoperative.
Also, with the present systems, the time allotted for repair of the component cooling system and the service water system is usually limited to the reactor refueling time during which all of the fuel assemblies from the reactor core are also in the spent fuel pool. Such time is on the order of twelve hours and can be inadequate.
There exists a need to provide a redundant cooling system in order to permit repair of the main cooling system or to act as a standby system.
To provide a redundant water-water heat exchanger to handle the cooling load in the event of such a failure or problem or during refueling is considered to be prohibitive not only because of cost and space limitations, but also because a redundant water-water system would not provide protection against loss of service water or component cooling water systems.
The use of a conventional air cooled heat exchanger as a redundant cooler is also impractical because such an exchanger is relatively inefficient, and to provide the same cooling capacity as the water-water heat exchanger would require a large installation.
It is known in the art that heat exchangers which employ pipe coils through which the liquid to be cooled is to circulate and which are subjected to a flow of air, such as ambient air, into which a spray of water is directed, are more efficient in cooling than heat exchangers which use only ambient air without a water spray for cooling.
Compact plate-fin heat exchangers have been used, for example in aircraft. Such heat exchangers provide greater heat transfer surface per unit volume by the use of fins of very small cross section brazed or otherwise attached in good thermal contact with the primary heat transfer surface. The two principal arrangements employing extended surfaces are the plate-fin exchanger, which has no pipe coils, and the tube-fin exchanger. The primary heat transfer surface of the plate-fin design consists of multiple parallel plates connected by fins; the space between each pair of plates comprises a fluid passage. Alternate fluid passages are connected in parallel by suitable headers to form the two "sides" of the heat exchanger. In the strip-fin type of plate-fin exchanger, metal strips arranged either staggered or in-line serve as the fins.
Tube-fin heat exchangers have fins on only one side of the primary surface, and tubes (either round or flattened) placed through holes in thin metal plates and to which the tubes are brazed. The preferred heat exchanger of the present invention has fins on both sides of the heat transfer surface and employs no tubes.
U.S. Pat. No. 4,969,507 describes a falling film air-cooled surface condenser in which droplets of coolant liquid are detached from the falling film of water and entrained by a flow of coolant air, enhancing heat exchange.
It has now been found that a heat exchanger of reasonable size and cost and which uses, as the coolant, ambient air into which a spray of water is directed concurrently with the flow of cooling air, can be used not only to substitute for the conventional water-water heat exchanger in the event the latter becomes inoperative, but also to provide component and spent fuel cooling in the event of failure of the water supply to the component cooling system, and hence, to the spent fuel water-water heat exchanger.